The present invention relates to a method of and apparatus for increasing data rate in a communication systems comprising at least one central station and a number of remote units and in which receiver channel estimation is aided by including known data in the transmission. The invention is applicable to, but not limited to the Global System for Mobile communication (GSM) cellular network.
Communication systems exist in which receiver channel estimation (including phase and timing synchronisation as a special case) is performed by including known data in the transmission, in the following referred to as training data. Typically these systems use a predetermined training data structure which is common for all users and in all environments. As a consequence it is necessary to design the training data to operate satisfactorily in the worst case situation. A training data structure typically consists of sequences of predetermined training symbols inserted in the data stream at predetermined intervals.
As an example, the GSM cellular communication standard includes predetermined training data in form of a midamble in all data bursts. This midamble has been designed so that it will provide suitable channel estimation for the worst case environment specifically in terms of the maximum delay spread and minimum time coherency. The typical GSM receiver demodulates data by performing a new channel estimate for every new burst regardless of the channel estimate for the previous burst. This is necessary for a fast moving mobile in an urban environment where the burst to burst coherency is negligible. For other environments, such as the low speed indoor environment, the burst to burst coherency is quite significant and the delay spread negligible. Communication systems such as GSM however include the same training data irrespectively of the specific requirements in the given environment. Furthermore, the receivers utilise the midambles for reception regardless of the actual propagation conditions.
As a consequence these communication systems end up including training data sufficient for the worst case propagation conditions and Bug channel estimation or synchronisation algorithms relying on this information always being present. The training data typically take up a significant proportion of the channel capacity thus causing a significantly reduced user information data rate. As an example the overhead associated with midambles in GSM is 23% of the data rate.
This approach is very wasteful in terms of data capacity and there is a need for an improved method of operation of the communication system.
European Patent Application Publication EP-A-0 615 352 (Nokia Mobile Phones Ltd) discloses a radio phone system utilising an adaptive training sequence whereby the length or structure of the training sequence is altered. However the radio phone system requires that both receiving and transmitting stations are compatible with an adaptive training sequence and therefore cannot be used far example with a GSM system which includes standard GSM handsets as these rely on a fixed training sequence.
Accordingly, the invention seeks to provide an increased data rate throughput for a communication system by more flexibly structuring the distribution of user data and training data.
According to a first aspect of the invention, there is provided a method of increasing a data rate in a communication system operating in a radio environment with certain propagation characteristics using repeated predetermined training data (205) and including a first central station (101) and a first remote unit (103) with certain training data requirements, and a second remote unit (107) with different training data requirements, and said first central station (101) and said first and second remote unit (103,107) each containing a receiving unit and a transmitting unit for communication over radio channels (105), the method comprising the steps of: a) determining (303) the propagation characteristics; b) determining (305) a training data structure; c) transmitting using said training data structure; and d) receiving using said training data structure said method being further characterised by the steps of e) determining (301) the training data requirements of the first remote unit and the second remote unit; and
the determined training data structure being dependent on the determined propagation characteristics and the determined training data requirements.
The communication system may be a GSM cellular communication system and the repeated training data a midamble. Preferably, the midamble structure is determined from standard GSM measurements.
According to a different aspect of the invention there is provided An apparatus for a communication system operating in a radio environment with certain propagation characteristics using repeated predetermined training data (205) and including a first central station (101) and a first remote unit (103) with certain training data requirements, and a second remote unit (107) with different training data requirements, and said first central station (101) and said first and second remote unit (103,107) each containing a receiving unit and a transmitting unit for communication over radio channels (105), the apparatus comprising a) means for determining the propagation characteristics; b) means for determining a training data structure used by the transmitting units and the receiving units, said apparatus being further characterised by comprising means for determining the training data requirements of the first remote unit and the second remote unit; and by the determined training data structure being dependent on the determined propagation characteristics and the determined training data requirements.
According to this invention the training data structure can be matched to the training data requirements of the individual remote unit and the current propagation environment, and it is thus not required that the training data structure is determined by the worst case conditions.